Asymmetrical AC trigger simulation

ABSTRACT

The present invention teaches a dimmer which incorporates an alternating current (AC) trigger which exhibits asymmetrical electrical characteristics. This eliminates the undesirable snap on hysteresis effect associated with conventional dimmers utilizing symmetrical AC triggers such as diacs and silicon bilateral switches (SBS). One embodiment of the present invention utilizes a zener diode to create the asymmetry. During one polarity of the AC source, the breakover voltage of the trigger is increased, forcing the trigger to breakover at a time later in the AC cycle than it would otherwise have with a symmetric trigger. This compensates for charge dumping of the phase control capacitor into the gate of the triac which would otherwise cause the snap on hysteresis effect.

This Application is a continuation of U.S. patent application Ser. No.08/183,459 filed Jan. 18, 1994 and now U.S. Pat. No. 5,619,081 issuedApr. 8, 1997.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of electric lightdimmers and controls, and more particularly to a system for controllingand eliminating undesirable characteristics associated with conventionalelectric light dimmers, such as what is known as snap on hysteresiseffects.

Conventional low cost light dimmers of the type a consumer mightpurchase from his or her local hardware or mass merchandising storepresently exhibit an undesirable characteristic which will herein bereferred to as the snap on hysteresis effect. This effect manifestsitself in the operation of a dimmer by causing the lamp to which thedimmer is connected (such as an incandescent electric light bulb) toturn on at an initial brightness level somewhat and often significantlyhigher than the minimum brightness level achievable. This effect is bothunexpected and undesirable.

Typically, dimmer designs today incorporate semiconductor devices thatperform the dimming function, and comprise an AC switch and a triggercontrol circuit to control the AC switch. Conventional trigger circuitsemploy diacs connected to the gate of a triac which acts as the ACswitch to turn the triac on and turn off. A phase shift circuit usedwith the diac determines where in the half cycle of the AC voltagesupply wave the triac fires, thereby determining the duration of timecurrent flows through the lamp which, in turn, determines itsbrightness. A typical phase control circuit of the type presently beingsold by Leviton Manufacturing Co., Inc. of Little Neck, N.Y., forexample, utilizes a trigger that exhibits symmetrical electricalcharacteristics during both positive and negative half cycles of the ACvoltage. Due to characteristics of the triac, however, the phase controlcircuit exhibits asymmetrical electrical characteristics whencontrolling the gate of a triac. The end result is that the triactriggers earlier in the half cycle than it would otherwise have and at ahigher than minimum brightness level. A user, when confronted with thehigher than desired brightness level, will back off the control (turnthe control knob, for example, in the dimming direction) to achievebrightness levels closer to the minimum achievable level.

In addition to the undesirable higher initial brightness level of thelamp, another drawback of the snap on effect is that if power isinterrupted and the brightness was backed off after initial turn on, andthereafter the power was restored, the light might not come on at all.Furthermore, if the phase control circuit utilizes a series of steppedresistances instead of a potentiometer, it is not possible with suchconventional devices to reach relatively lower brightness level.

Prior art attempts to solve this snap on hysteresis problem haveincluded utilizing two circuits to control the firing of the triac,rather than a single circuit. The first such circuit controls the timing(i.e. brightness) of firing, while the second circuit controls thecharge dumping of the capacitor into the gate of the triac. Drawbackswith this prior art approach include an increase in cost and addedcomplexity of the overall circuitry.

Another attempted solution to the snap on hysteresis problem is to usean asymmetrical trigger in the phase control circuit. This can be doneusing a number of discrete components or using AC triggers havingasymmetrical electrical characteristics. The resulting devices, however,are not believed to be commercially available and having to add a numberof components to a phase control circuit design to achieve the effect ofan AC trigger increases its cost and complexity.

The result has been a long felt need for a solution to this problem thatis simple and effective, and yet costs very little in terms of componentcost and circuit complexity.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide an AC triggerhaving asymmetrical electrical characteristics so as to eliminate thesnap on hysteresis effect.

Another object of the present invention is to provide such an AC triggerwhich is relatively inexpensive.

Yet another object of the present invention is to provide an AC triggerwhich is simple and relatively easy to incorporate into circuits.

Other objects and features of the invention will be pointed out in thefollowing description and claims and illustrated in the accompanyingdrawings, which disclose, by way of example, the principles of theinvention, and the best modes presently contemplated for carrying themout.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings in which similar elements are given similar referencecharacters:

FIG. 1 is a circuit diagram of a conventional dimmer of the type foundin the prior art;

FIG. 1A is a graph illustrating typical electrical characteristics of asymmetrical trigger as found in the prior art;

FIG. 2 is a circuit diagram of a dimmer constructed according to theconcepts of the present invention;

FIG. 2A is a graph illustrating the electrical characteristics of thedimmer of FIG. 2;

FIG. 3 is a circuit diagram of a further embodiment of a dimmerconstructed according to the concepts of the present invention;

FIG. 4 is a circuit diagram of yet another embodiment of a dimmerconstructed according to the concepts of the present invention;

FIG. 5 is a circuit diagram of still another embodiment of a dimmerconstructed according to the concepts of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In order to provide,the reader with a more complete understanding of thepresent invention and an appreciation of its advantages, a descriptionof a preferred embodiment of the present invention in a typicaloperating environment is presented below.

Shown in FIG. 1 is a schematic showing a conventional triac phasecontrol circuit as found in the prior art and used extensively indimming and other applications such as motor speed controls. Thecharacteristics of its symmetrical trigger are shown in FIG. 1A.Conventional low cost dimmers typically employ this type of circuit. Itsuffers, however, from a feature called snap on hysteresis. At the startof a positive half cycle, assuming the dimmer shown in FIG. 1 isenergized, and the potentiometer 16 is set to maximum resistance, theload 12, typically a lamp, will be off. This is because the peak voltageacross the capacitor 18 does not exceed the symmetrical trigger or diac20 breakover voltage V_(B). As the resistance of potentiometer 16 isreduced, a point will be reached where the capacitor 18 voltage exceedsthe diac 20 breakover voltage V_(B). When the diac 20 breaks over,charge from the capacitor 18 flows into the gate of the triac 22 to turnit on. At the same time, the capacitor 18 voltage drops to a value lesspositive than before the reduction in charge. As shown in FIG. 1A, thesymmetrical trigger or diac 20 exhibits negative resistance as it startsconducting more current. Thus, as the triac starts conducting, thevoltage across the diac 20 drops. This allows more charge to flow fromthe capacitor 18 into the gate of the triac 22. At this point, duringthe following negative half cycle of the AC source 14, the diac 20breakover voltage-V_(B) is reached sooner because the capacitor 18 has areduction in maximum positive voltage due to its loss of charge in thepositive half cycle. The result is that subsequent diac 20 trigger orbreakover points, both positive and negative, occur earlier than thediac 20 breakover point of the very first half cycle. Triggering thediac 20 sooner in the AC cycle causes the light to turn on at a highervoltage. This results in the light 12 being brighter than that which thefirst half cycle would have provided. In order for a user to dim thelight, the potentiometer 16 resistance must be increased. It isundesirable to require a user to reduce brightness after turning on thelight.

The solution of this problem afforded by the present invention is toprovide an asymmetrical trigger or diac. FIG. 2 shows a block diagram ofa dimmer 58 incorporating an asymmetrical trigger 10 connected betweenthe phase control circuit consisting of potentiometer 24 and capacitor26 and a triac 28. The trigger characteristics of the asymmetricaltrigger 10 are shown in FIG. 2A. FIGS. 3 to 5 show alternativeapplications of the asymmetrical trigger 10 in dimming circuits.

Referring to FIG. 2A, in one polarity, the breakover voltage V_(B1) isthe same as in a symmetrical diac. In the other polarity, however, thebreakover voltage V_(B2) is increased by the same amount as the voltagedifference between breakover and maintenance in the first polarity,V_(B1-V) _(M). The result is that even though the capacitor 26 voltageis reduced during the first half cycle, it must charge to the higherbreakover voltage, thus maintaining the brightness level of the firsthalf cycle.

Since an asymmetrical trigger device as a single component is notavailable, its characteristics have been simulated with the presentinvention. This has been done using a zener diode 34 in series with asilicon bilateral switch (SBS) 36, as shown in FIG. 3. The zenerbreakdown voltage is chosen to be the difference between the breakoverand maintaining voltage of the SBS. Assume initially the capacitor 32 isfully discharged, with the potentiometer 30 near its maximum setting,the capacitor 32 begins charging. When the capacitor 32 voltage reachesthe breakover voltage of the SBS 36, it starts conducting and currentflows through the zener 34, which is forward biased, into the gate ofthe triac, turning it on. Ass current flows into the gate of the triac38, the voltage across the SBS 36 drops to its maintaining voltage dueto its negative impedance characteristics. As in the case of a diactrigger, this causes additional charge to be removed from capacitor 32.In the following half cycle the zener 34 is now reverse biased and willnot conduct until its breakdown voltage is reached. The zener 34breakdown voltage, however, is chosen to be the difference between thebreakover and maintaining voltage of the SBS 36. Therefore, thecapacitor 32 must charge an additional amount; the value of the zener 34breakdown voltage. Thus, utilizing an asymmetrical trigger causes theSBS 36 and the triac 38 to trigger later in the AC cycle than it wouldhave with a symmetrical trigger, thereby causing the elimination of theundesirable snap on hysteresis effect.

In an alternative embodiment of the present invention, an approach isused in conjunction with a conventional symmetric diac 46, as shown inFIG. 4. As with the design utilizing the SBS 36, the requirement here isthat the zener 44 breakdown voltage equal the difference the breakoverand maintaining voltage of the diac 46. The operation of the device ofFIG. 4 is substantially similar to that described with respect to theSBS 36.

In yet another embodiment of the present invention, the desired resultis achieved utilizing a quadrac semiconductor device 56, as shown inFIG. 5. The quadrac consists of a triac with a diac connected in serieswith the gate of the triac. The operation of the device of FIG. 5 issubstantially the same as described above with respect to the SBS 36 ofFIG. 3.

The same benefits which have been described above utilizing dimmingcircuits as examples of the environment for the present invention mayalso be achieved using a triac based motor speed control which uses acircuit similar to that of a dimmer.

While there have been shown and described and pointed out thefundamental novel features of the invention as applied to the preferredembodiment, it will be understood that various omissions andsubstitutions and changes of the form and details of the deviceillustrated and its operation may be made by those skilled in the art,without departing from the sprit of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

What is claimed is:
 1. A dimmer comprising:a) switching means havingfirst and second terminals and a gate terminal; said first terminalconnected to one side of a load; said second terminal connected to asource of AC voltage; said switching means operable to change from astate of high impedance to a state of low impedance upon the applicationof sufficient current to said gate terminal causing current to flowthrough said load; b) a conductor coupled between the other side of saidload and said source of AC voltage; c) symmetrical trigger means havingthird and fourth terminals, said symmetrical trigger means fourthterminal coupled to said gate terminal of said switching means forpreventing said switching means from changing state until sufficientvoltage appears across said third and fourth terminals of saidsymmetrical trigger means; said symmetrical trigger means exhibitingnegative impedance characteristic upon breakover and having similarbreakover points during both positive and negative half cycles of asource of AC voltage; d) phase control means coupled between said oneside of said load, said source of AC voltage and to an output terminal;and e) asymmetrical trigger means connected between said output terminalof said phase control means and said third terminal of said symmetricaltrigger means, said symmetrical trigger means operable to compensate forcharge depletion in said phase control means caused by said symmetricaltrigger means; said asymmetrical trigger means effective to change stateasymmetrically during each half cycle of said AC voltage.
 2. A dimmer,as defined in claim 1, wherein said phase control means furthercomprises:a) potentiometer means having a fifth and sixth terminal, saidfifth terminal coupled to said one side of said load to control thecurrent flowing to said load from said source of AC voltage; b) acapacitor having seventh and eighth terminals, said eighth terminalconnected to said source of AC voltage; and said seventh terminal ofsaid capacitor coupled to said sixth terminal of said potentiometer andto said asymmetrical trigger means.
 3. A dimmer, as defined in claim 1,wherein said asymmetrical trigger means includes a silicon bilateralswitch.
 4. A dimmer, as defined in claim 1, wherein said asymmetricaltrigger means includes a diac.
 5. A dimmer, as defined in claim 1,wherein said asymmetrical trigger means includes a quadrac wherein adiac of said quadrac comprises said symmetrical trigger means.
 6. Adimmer, as defined in claim 1, wherein said asymmetrical trigger meansincludes a zener breakdown diode.